Best Render Settings to Eliminate Image Grain?
The Intel Denoiser might be useful here.


Check Denoising Data in Properties Panel > View Layer
Check Use nodes in Compositor
Add the Denoise node and connect it like this


Now render your image and it will be automatically denoised.  



Note:
If you're not going for photorealism, you'll be better off using Eevee Render Engine.
It can be changed in Render Tab > Render Engine > Eevee